Water level indicators having a remote fibre optic readout display are well-known. An example of such a device may be seen in U.S. Pat. No. 3,713,338. In such devices the water level optical signal is focused on one end of a fibre optic assembly and is transmitted along a significant length of the fibre optic assembly to a remote readout area where the other end of the fibre optic assembly terminates.
In certain situations a local readout of the water level signal is desired in addition to the remote readout display. In the past, to provide such a local readout, a short fibre optic assembly was mounted side-by-side with the remote readout fibre optic assembly and the optical signal was focused on both of the fibre optic assemblies. The short fibre optic assembly thus provided the local readout while the second fibre optic assembly of significant length terminating at the remote location provided the remote readout. Applying this system to measure the water level of a boiler required the extended length remote readout fibre optic assembly to extend up to 330 feet to the control room of the power generating plant. The short fibre optic assembly was relatively short since it terminated right at the boiler drum to provide the local readout of boiler water level.
Certain problems arise with such side-by-side fibre optic assemblies to provide local and remote water level readout. Since the same optical signal must be collected by both of the side-by-side fibre optic light guide assemblies, the focal spot which illuminates these assemblies must have optimum size in order that 50 percent of the incident light enter each fibre. Furthermore, thermal variations generated dimensional changes in the mounting hardware of the fibre optic pair further requiring that the focal spot of the optical signal be even larger in area than otherwise required to cover the fibre optic pair to compensate for the thermal induced movements. This resulted in a weak optical signal to the fibre optic pair which signal was then further diminished in the long length loss of the remote readout fibre optic.